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Ambient occlusion is an extremely useful way to get realistic shading on your model without expensive Final Gather and Global Illumination calculations. Although ambient occlusion does not give you color bleed, combined with careful lighting it can produce wonderful results! Ambient occlusion is usually used in the composite, but applying the layer with a multiply function. Also of note, if you have a lot of specularity in your render, it's worth rendering out a separate specular pass, and compositing this over the ambient occlusion pass so you don't muddy up your highlights.